Various methods have been known for the production of foundry sand molds (hereinafter referred to simply as sand molds) to be used for producing various metal cast articles. However, they can be generally classified into the following two methods depending upon the types of the binders used. Namely, a method for solidifying foundry sand (hereinafter sometimes referred to simply as sand) composed of e.g. coarse particles of silica sand or zircon sand having a particle size of e.g. at least 325 mesh by an organic binder, or a method of solidifying such sand by an inorganic binder.
Among them, as the former method of using an organic binder, methods of using a resin and a curing agent thereof in combination are is known, including, for example, a method in which a phenol resin or a furnace resin mixed to sand is cured by a strongly acidic curing agent such as sulfuric acid, phosphoric acid, toluene sulfonic acid or xylene sulfonic acid to solidify the sand, a method (a pep set method) wherein three components of a phenol resin, a polyisocyanate and a basic catalyst, are mixed with sand, and the reaction of the phenol resin with the polyisocyanate to form urethane is initiated by this catalyst to solidify the sand, and a method (a renocure method) wherein sand is solidified by utilizing a urethane-forming reaction which takes place by the mixing of three components of an oil-modified alkyd resin, a metal salt of naphthenic acid and a polyisocyanate. As the latter method of using an inorganic binder, a method (an OJ process) wherein a casting mold is formed by solidifying sand with cement, and a method wherein sand is solidified by injecting CO.sub.2 gas to sand having sodium silicate contained therein, have been known.
However, in each case, the sand mold prepared by such an organic binder is usually inferior in the strength after pouring a molten metal (hereinafter referred to as "high temperature strength"), and once a molten metal is poured into such a sand mold, the organic binder will burn, and the bond of sand particles tends to loosen, whereby the molten metal will penetrate into sand to cause a so-called penetration trouble To prevent such penetration of the molten metal into the sand mold, it is necessary to apply a mold coating agent containing as a mold coating material graphite, mica powder, charcoal, talc or the like as the main agent, to the portion of the sand mold which will be in contact with the molten metal, by a brush or spray. This mold coating operation constitutes from 30 to 50% of the production cost of the sand mold and thus was a main factor for the cost up of the sand mold.
With respect to this problem, Japanese Examined Patent Publication No. 40617/1988 proposes a material for a mold which is excellent in the high temperature strength and the strength after being left to stand still in the atmosphere at room temperature for a predetermined period (hereinafter referred to as "stand still strength") and which requires no mold coating or a simple mold coating, by adding two components of a ceramic binder (one or more members selected from tetraalkoxysilanes, their hydrolysis.dehydration.polycondensation products, water-dispersible silica sols and alcohol-dispersible silica sols) and a polyisocyanate as a curing agent for this ceramic binder at the time of the preparation of a sand mold by means of an organic binder and its curing agent.